Signaling system



May 26, 1942. H. s. BLACK 2,284,555

SIGNALING SYSTEM Filed July 50, 1940 2 Sheets-Sheet l May 26, 1942. H.s. BLACK 2,284,555

SIGNALING SYSTEM Filed July 30, 1940 2 Sheets-Sheet 2 FG' 3B, (L F/G.3c

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Patented Mayl26, 1942 l SIGNALTNG SYSTEM Harold S. Black, Elmhurst, N.Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application July 30, 1940, Serial No.348,433

8 Claims.

This application is a continuation in part of my application, Serial No.114,390, led December 5, 1936, (Patent 2,209,955, August 6, 1940), forWave translation systems, and relates to signal transmission, especiallytransmission` over long circuits.

Representative objects of the invention are to reduce transmission timeof such circuits, as, for example, loaded telephone or telegraph lines,and speed up transmission of signals and reduce their distortion.

In accordance with the invention, a negative feedback amplifier may beconnected to the receiving end cf such a line and the feedback path maybe given characteristics suitably related to those of the line, aspointed out hereinafter, to reduce signal distortion, as, for example,reshape telegraph signals, and to speed up tranmission, as,'forinstance, by preventing loading coils from increasing the tranmissiontime of the line.`

A feature of the invention relates to dividing a corrective amplifierinto tandem portions, each a negative feedback amplifier, to facilitateob taining stability against oscillation.

A further feature relates to Vcompensating for cable modulation, due tonon-linear loading elements or magnetic shielding, for example, byintroducing similar modulation producing elements in the feedback pathof a corrective feedback amplifier amplifying the signals from thecable. i'

Other objects and aspects of the invention will be apparent from thefollowing description and Fig. 5 shows a submarine telegraph cablesystem embodying a form of the invention; and

Fig. 6 shows a transmission system embodying a corrective amplifiercomprising feedback amplifiers in tandem.

The amplifier of Fig. l may be a stabilized feedback amplifier of thegeneral type disclosed, for example, in the copending applicationmenticned above, or my Patent 2,102,671, December 21, 1937, or my paperon Stabilized feedback amplifiers," Electrical Engineering, January,1934. It comprises an amplifying path or element shown as includingtandem connected vacuum tubes i and 2, and comprises a feedback y path fshown as including a transmission control network 3 of generalizedimpedances. The

amplifying path or element may be referred-to as the a-circuit, and thefeedbackpath may be referred to as the -circuit. the significance of aand p being as indicated in the Patent 2,102,671, just mentioned. Thenetwork 3 may be referred to as the -circuit network.

An input hybrid coil 5 couples the incoming i. e., the impedance 9 orlli across the bridge` points of the hybrid coil.

The network 3 may be a -circuitnetwork for amplitude or phaseequalization or correction of distortion for a section of cable orcircuit in front of the amplifier. Locating the corrective network inthe feedback path or p-circuit instead of ahead cf the amplifier has animportant advantage relating to speeding up transmission. As shown in myabove-mentioned Patent 2,102,671, the amplification of a feedbackamplifier with a 1 is approximately so when the p-circuit is made auchthat its.

propagation is the same as that of the line or cable between a source ofvoltage e and the input to the a-circuit, the transmission from thesource of voltage e to the output ofthe u-circult is (as brought out,for example, in the patent just mentioned or in my Patent 2,002,499, May28, 1935, or F. A. Cowan Patent 2,017,180, October l5, 1935), and hencethere is no delay nor distortion and the amplified signal appearsinstantly at the output of the /i-circuit as a replica of the signal eapplied to the line or cable by the source, except. reversed in sign.(There is no restriction on other than that the amplifier comply withNyquists rule.) This result is obtained from theoretical considerations.The equations which are available for arriving at this conclusion arerigorous only for systems containing lumped constants. The transmissionthrough systems made up of continuous elements is only approximated fromthe lumped constant equations. Hence to this extent the above procedurewill correct for amplitude distortion and phase distortion.(Applications are encountered where there are likewise importantadvantages in having the -circuit correct solely for phase distortion.)f y Compared to the customary way of improving cable distortion thisprocedure speeds up transmission. The customary way, where the networkfor correcting attenuation and phase over the frequency band of interestis not in a feedback path, delays the time of transmission by a periodthat exceeds the time required for a particularA frequency to traveldown the cable, this particular frequency generally being that one inthe band of interest which is most slowly transmitted. In contrast tothis, for the described procedure of correcting in the -circuit. thetime is the time for a particular frequency, which is the one mostrapidly transmitted, or in other words, it is the time required forcurrent no matter how trivial to make its appearance at the receivingend. All other velocities for all other frequencies for which thecircuit is properly operative are made equal to this most rapid speed.In the case of a cable, this speed apparently corresponds to a speedless than the velocity of light l in a vacuum, depending upon thedielectric constant and permeability of the cable. It should be notedthat this time is independent of the Wave form impressed at the sendingend. It is also independent of lumped series nductance, either positiveor negative, and likewise is independent of lumped shunt resistance,either positive or negative. Thus, by adding apparatus (the feedbackamplifier) to a heavily loaded voice frequency cable, a waveY can bepropagated over the cable at the same speed as over the same cablenon-loaded. The circuit indicated at `6 in Fig. 2 may be such a cable,for example a telephone cable fed from a source Il of voice currents andperiodically loaded with series impedance, for example, seriesinductance or shunt impedance, for example, shunt resistance, or both,the system of that ligure being .otherwise similar to the system of Fig.1 except that. in place of the -circuit network 3 of Fig. 1 a speciiicform of -circuit network 3', about to be described, is shown. However,if desired, the network 3 may be replaced by a replica of theperiodically loaded circuit 6, for example to reduce effects of cablemodulation as pointed out identically at the receiving end for likeexcita` tions at thesending end. If two systems have like indicialadmittanoes their steady state ain-f'v plitude and phase characteristicsare identical. If two systems have Vlike amplitude andphase.characteristics (that is, steady state attenuation and phase versusfrequency characteristics) over a specified band of frequencies, forexample f1 to f2, then the received currents lwithin this band K, nitenumber of lumped constants as shown at 3' .in Fig. 2 whose four elementsare two impedances Zzeach equal to the cable impedance of length Z/2with far end short-circuited and two other impedances Z1 each equal tothe cable impedance of length l/2 with far end open. That is, Z2 is theshort-circuit impedance, and Z1 the open-circuit impedance, of a lengthl/2 of the loaded cable). The lattice network can be made to match theloaded cable over a specified band as regards steady state amplitude andphase, and yet have the time for any or the first current to arriverelatively short compared to the case of the loaded cable.

The real time of transmission of a cable will be designated r and equalsl/v where v is the velocity of propagation referred to above, i. e., 1is the time for the first current to appear at the distant end.

When a feedback amplifier is to correct for a section of cable in frontof the amplifier in the general manner referred to above, preferably the-circuit should have an indicial admittance equal to the indicialadmittance of the cable when, referring to the admittance characteristicof the latter, t is replaced by (t-r). In other words, for the -circuitthe indicial admittance characteristic (i. e., the graph of current as afunction of time at the amplifier input end of the ,f2-circuit inresponse to unit voltage suddenly applied at time t=0 at the other endof the -circuit, with a=0) preferably should be the graph orcharacteristic that would be obtained by shifting the indicialadmittance characteristic of the cable section horizontally toward thecurrent axis an amount f, the amount necessary to make the displacedcharacteristic pass through the origin. Such indicial admittancecharacteristic for the -circuit will be obtained if, except for leavingout a uniform delayl r, the -circuit be given the same steady stateattenuation and phase characteristics as the cable section from zerofrequency to infinite frequency and be vso constructed of lumpedimpedances as to have substantially zero transmission time compared tothe transmission time for the cable section. Then the output of theamplifier will be a replica of the input to the cable section at thesending end except it will appear r seconds later. As a practicalmatter, such amplifier output can be obtained by constructing the-circuit network as described above for network 3', with its steadystate attenuation and phase characteristics substantially the same asthose of the cable section over a finite frequency range correspondingto the band of frequencies to be transmitted.

If, on the other hand, the -circuit be a sec-- itself)` the time oftransmision is still -r but for .an interval r at the beginning andacorresponding interval at the end, the wave form apparently will beincorrect. This effect would be something more than multiplying the timeof transmission by two as compared to the method of the precedingparagraph. It would make the received wave distorted in a manneranalogous to that of a loaded cable. In other words, the received Waveor wave delivered by the amplifier would not only be zero for aninterval r after the application of the excitation at the sending end ofthe cable, but would be a distortion wave, or wave of form differentfrom the applied signal for a further interval of f; and aftertheremoval of the excitation would not only continue for an interval f,but for a further interval r would a distortion wave or wave of formdifferent from the applied signal.

To give a physical concept of the methods described in the two precedingparagraphs for correcting phase distortion and speeding up transmission,the response without feedback to the excitation can be viewed as theforced or steady state response plus the free flow or transientresponse. As a result of feedback action the amplifier does not amplifythe transient response for the conditions above described. Thus, the rstcurrents to arrive of necessity carry information as to the signalimpressed so that the amplifier sends out a copy of the signal wave formapplied at the sending end, and, when the main body of the transmittedsignal arrives later and appears at the input to the amplifier, it justis not, amplified.

'I'his concept may be made more readily apparent from consideration ofthe transient solution of a simple system, as for example, the systemshown in Fig. 3 illustrative of the procedure of correcting phasedistortion of a line by terminating it in a feedback amplifier havingits -circuit a replica of the line. (In general, the application of avoltage in a circuit results `in a forced response and, in addition,natural or free responses. These free responses are considered thetransient response. In other words, the total current flowing is the sumof the forced response or steady state plus the transient or freeresponse. In evaluating transient response, as in determining the steadystate in my Patent 2,102,671, mentioned above, the finite velocity ofpropagation around the ,ri-path has not been taken into account. Thistacitly assumes an equivalent circuit involving lumped constants treatedin accordance with conventional circuit theory. It should be appreciatedthat conditions may be encountered in practical applications where itwill not be feasible to neglect the finite velocity of propagation. Ithas further been assumed that all parts of the system are linear and thevalues of all circuit elements, resistance, inductance and capacitance,are independent of frequency.) Fig. 3A shows the system of Fig. 3without the corrective feedback amplifier whose equivalent circuit isgiven in Fig. 3. The system without the feedback amplifier comprises aninductance in series with two resistances. For the system of Fig. 3 thetransients of the line circuit (which would also be the transients inZL=R/2 of Fig. 3A) and the transients of the feedback amplifier can bedetermined separately for the reason that the grid impedance is assumedto be infinite and so does not couple the two meshes.

Provided that a is not zero and assuming there is no other couplingpatlrithus assuming as zero the current usually neglected) between theinput and output of the amplier, then it can be shown that for the linetransients or free response of the line circuit, up is infinite andtherefore the line transients are not amplied by the feedback amplifier.Referring to the output of the amplifier which depends upon aV, thetransient that was characteristicv of the line circuit, which in thiscase is a transient characteristic of a resistance in seres with aninductance, has disappeared and in its place is a transient that ischaracteristic of the entire feedback loop. In this simple case, itsrate of decay is increased directly as the value of (l-afn for zerofrequency. Thus, the line circuit transient has vanished and theamplifier transient substituted can be made as short in duration asdesired merely by increasing a, but its magnitude relative to the steadystate input cannot be altered. Results of computations which took R as1200 ohms, L as 0.2 henry, and a as 2000+J0, are given in Figs. 3B, 3Cand 3D for an applied signal voltage E(t) taken as a telegraph dotapplied for microseconds, these figures showing this applied signal andthe computed value of signal received for this applied signal, with andwithout the corrective feedback amplifier. Fig. 3B shows this appliedsignal. Fig; 3C shows the received current ii., i. e., the currentreceived from the line, as computed for the case in which the correctivefeedback amplifier is not used, i. e., the case of Fig. 3A. Fig. 3Dshows the negative of the received current, in with the correctivefeedback amplifier in circuit, as in Fig. 3, or, in other words, exceptfor a reversal in signal, shows the current received by the loadimpedance, ZL=R/2. In the case of Fig. 3, with the specific circuitconstants given above the (computed) negative of the received current,-ir., builds up to 0.95 of its steady state value in one microsecond.Likewise, it takes but one microsecond to similarly reduce the current.

Fig. 3E shows the free response of the line, the forced response of theline, and the total current delivered by the line, this latter currentbeing the sum of the other two, that is, the sum of the forced responseor steady state and the transient or free response. This figureindicates how, since the amplifier gain is zero for the transient of theline, the only portion of the input to the amplifier from the line thatis amplified or appears in amplified form in the amplier output is thesteady state or forced response.

A point to note in connection with Fig. 3D is that with the correctivefeedback amplifier the output wave is practically undistorted, that is,of practically the same form as Ed), (the applied voltage shown in Fig.3B), and, furthermore, the extent to which it is distorted can be madeless than any preassigned value merely by increasing a. The examplepresented `also gives a physical concept of how, at the beginning of thepulse, although the input to the amplifier is practically zero, theoutput current jumps up as rapidly as desired and starts right off inits steady state or undistorted form, and, at the end of the pulse,although the voltage persists across the input to the amplifier for along time after the excitation at the sending end of the line has beenmad-e zero, the output of the-arnpliiier immediately drops to zero asquickly as desired provided a is made suiiiciently large. The physicalconcept is that by making the pcircuit a copy of the line circuit,A thefree or sion and reshapes the signal.

sideration, are not amplified. The feedback amplier, however, does setup its own transients, both at the beginning and at the end, and thepractical duration of Athese emplifier transients is reduced byincreasing llil,

`A corrective passive network to take the place of the correctivefeedback amplifier in such a system would have to give an effectequivalent to insertion of anegative inductance of value "-L and zeroresistance, and do so over a broad band of lfrequencies, especially if,rfor the case ofthe amplifier, be increased and the durationy of ,thepulse consequently shortened.

' If; for a system comprising an amplifier with y,..,se 1`a.nd with the-circuit of the amplifier a replica of the line or circuit ahead of theamplifier, the transmission from the source to the amplifier linput wereand the transmission from the ampler input `to the amplifier output werethe over-all Vtransmission from source-to amplifier output would be andthis would correspond to perfect transmission at infiite speed.Practically, the speed is limited ,to that corresponding to a timeextending from the instant a voltage is applied at the sending end ofthe line to the instant any resulting voltage,- no matter how small,appears at the amplifier input, plus the electron transit time requiredfor electrons to travel from the cathode ato the plate of the tube, etc.VWhen a force or a small current to rst make its appearance at thereceiving end, or vin other words, adding lumped positive or negativeinductance does not affect the speed of the system being described;whereas it is a matter of common experience that lumped loading and themethods of present practice greatly slow down the speed.

The amplifier reshapes the output signal so as to agree with the voltageor excitation E at the sending end of the line or circuit ahead of theamplifier (as indicated by Fig. 3D, for exexcitation is suddenly appliedto the sending end of the line or circuit ahead of the amplifier theresult is a transient anda steady state. With the ,z3-circuit of theamplifier a replica of the 'precedingline or circuit'the amplifierpositively does not amplify the transient at all, because for thetransient or free response a is infinite. There is a fillet or delay inbuilding up of the signal .output delivered by the feedback amplifierand also in cessation of the signal. This is the transient of the[L5-loop if it be opened (and properly terminated at each side of theopening) divided by (1 /Lp). Therefore by making a as large as requiredthis effect can be caused to be less than any Varbitrarily chosen value.Therefore the corrective feedback amplifier reduces the time oftransmission or speeds up transmis- The speed of propagation ortransmission of the signal is the speed with which the first currentmakes its appearance in response to the excitation produced Vat thesource (the output of the preceding repeater or input to the line). Thisspeed is independent of the wave form'of the excitation. The speed ofall frequencies is made equal to this value, whereas present practiceslows transmission down to a speedslower than the slowest velocity ofany frequency inthe band. (In a multiplex or multichannel system thespeeds of the slow channels are increased to the speed of the fastestpart of the fastest channel, whereas present day practice slows down thefastest to vequal theslowest part of the slowest and then 'adds 'some'additional delaym besides.) e Adding loading coils does not: alter" thetime required for ample); and a most remarkable property (due to thefeedback action) is that vit does this notwithstanding the fact that theinput voltage applied to it may have at all' times vaV wrong value. Itnevertheless delivers signals of the proper wave form to the outgoingcircuit before the wave it receives over the line at its input hasattained the steady state or the wave form of the excitation E; and itstops delivering output to the outgoing circuit when the excitation E atthe sending end of the line stops. It will stop thus even though atransient due to the excitation E may appear across the input of theampliiier for a time after E is short-circuited lasting a thousand timesas long as E lasted; and though the amplifier correctly refuses toamplify this transient wave it meanwhile will amplify with proper waveform any waves it should that may be applied at the sending end of theline during the time this transient appears across the amplier input.

In the case of a multistage amplifier, it can be shown fromconsideration of the transient solution that, with feedback, all of thetransient components appear in each of the meshes, that is, everywherein the p13-path. For example, it can be shown from evaluation of thetransient response in the case of the amplifier of Fig. 4 with specificconstants and with the applied wave a cosine wave of frequencyapproximately at the middle of the transmission or pass-band of theamplifier, that neither of the natural responses (free responses) of theamplifier without feedback appears in the amplifier with feedback.`

Instead, two new pairs of terms appear. One of them is a very highfrequency oscillatory vWave and the other a very low frequencyoscillation.

Without feedback it is possible for the transient response to exceed theforced response in magnitude at certain parts of the circuit.v Notably,in the case of transient solution of the circuit of Fig. 4 calculatedfor the specific constants referred to above, at the grid of the firsttube, with feedback the value of one of the free response terms exceededthe forced response by about 55 decibels. (This was with the capacity ofcondenser I2 equal to zero, or in other words with the condenserabsent.) Considering kthat the excitation was in the neighborhood ofmid-band frequency, the transient response evaluated is indicative ofwhat might occur due to abrupt changes in signal input. Consequently, ina high power amplifier care should be taken to avoid a condition oftransient overloading of the input tubev of this magnitude because a55-decibe1 transient overload, if the tube is to handle it,

corresponds to a power ratio of 316,00011, and to increase the powercapacityof even the second from the last stage bysuch an amount as thismay Y become a vvery practical consideration. However, the calculationindicated that there was no overloading of the last tube, 55 decibelsbeing the amount of mid-band frequency feedback so that the transient onthe grid merely equaled the voltage normally fed back. As indicatingthat this overloading of the input tube by transients depends upon thesteepness of the applied impulse, it is noted that when the suddenlyapplied voltage was assumed a sine wave instead of a cosine wave, therecomputed value of the free response across the grid of the first tubeof the three-stage amplifier exceeded the forced response by 26 decibelsinstead of 55 decibels.

To avoid overloading of the first tube by transients produced byfrequencies in the transmitted band it is desirable to (l) maintain thevalue of a considerably different from 1L 0, (i. e., keep the polar plotof a outside of a circle of considerable radius centered at the point l,and (2) at the same time provide a suitable capacitance across the gridand cathode, for example, by adding a condenser I2 that renders thecapacitance across the grid and cathode just sufficient to prevent undueoverloading of the first tube by the transients. Fullling condition (l)does not affect the amplitude of the transient,l

but reduces its duration so that, with condition (2) also fulfilled thetransient cannot last long enough to charge the capacity across the gridand cathode to a voltage so -high as to overload the tube. A stillfurther precaution that may be advisable for avoiding the transientoverloading is to maintain the phase of a as nearly as practicablebetween 90 and 180 degrees, i. e., keep the polar plot of a inthe'second and third quadrants as nearly as practicable.

With feedback, the original transients present without feedback areeliminated and new ones are produced which are characteristic of thefeedback amplifier. Of course, if the amount of feedback is not large,the new ones might not differ appreciably from the old ones because inthe limit, with no feedback, ri-20 and the two transient responses arealike. However, in the case of the feedback amplifier, all of thetransient terms appear in varying degree in every mesh of the amplifier,whereas in the non-feedback amplifier the transient terms appear only inthe mesh that generates its particular transient term or terms and insucceeding meshes.

As in the case with feedback, without feedback the first interstagecircuit generates a transient. That transient is amplified and appearsin the output. In the second interstage mesh, a second transient isproduced which in turn is likewise amplied and appears in the output.However, neither appears in the input.

On the other hand, with feedback it is impossiblev to ascribe anyparticular transient term to any particular reactance element or groupof elements and the complete transient performance of the amplifierdepends upon the entire number of reactances in the afi-path. This,therefore, differs from the non-feedback circuit where the transientresponse depends upon the free response of each mesh and each particularfree response can be definitely ascribed to the mesh so producing it.

Fig. 5 shows a system comprising a source I3 .shape the signals at thereceiving end of the cable, for actuation of the telegraph receiver I1,in accordance with 'the principles set forth above, (it being noted thatthe wave shown in of telegraph signals for transmission over a sub- 76Fig. 3C as a signal received without the corrective feedback amplifiermay be considered as giving an optimistic picture of the actualtelegraph problem).

In a practical signaling system transmitting, for example, over asubmarine cable as in Fig. 5, or over a land cable as in Fig. 2, (unlessthe corrective feedback amplifier is to be allowed to oscillate with theoscillation amplitude or amplitudes maintained below the overload valuefor the amplifier) the matter of making the amplifier satisfy Nyquistsrule for stability needs careful consideration, especially when the-circuit of the amplifier is made a replica of the cable or is given anindicial admittance equal to that of the cable. This is true for thecase of the submarine cable, especially if the submarine cable isv along cable. as for instance a transoceanic cable; and it is'true for thecase of the land cable, even though the land cable may be relativelyshort. To facilitate compliance with Nyquists rule, instead of oneamplifier with the propagation of its -circuit adjusted to the value twoamplifiers may be used in tandem, at the receiving end of the cable. forexample as shown at 2| and 22 in Fig. 6, with the products of thepropagations of their -circuits equal to which may equal the propagationof the cable. For example, designating the a and for one of the twoamplifiers as ai and si, and designating the a and for the other of thetwo ampliers as a: and pa, then, with a11 l and azn l, the amplificationof the two amplifiers in tandem and by making 1z=, the proper correctiveeffect is obtained. Obviously, the number of tandem amplifiers can beextended to n, e. g.,

Then, (compared to construction of a single amplier with of proper valueto make the amplifier produce the desired corrective effect yet satisfyNyquists rule) it is relatively easy to so make p1 and z. etc., as tosatisfy (l) and at the same time make jni, arpa, etc., obey Nyquistsrule, since the. total phase .shift required in the -circuits can bedivided between the -paths of the tandem connectedampliflers. In Fig. 6,the signal transmitter is shown at 25, the transmission line at 28, andthe signal receiver at 21. The signal transmitter 25 may bea telegraphtransmitter as in Fig. 5, with circuit 26 a submarine telegraph cablesuch as that of.' Fig. 5, for example. 0r, as a further example, thesignal transmitter 25 may be a source ci' voice currents, gith line 28atelephone line such as that of Fig.

In systems in which the -circuit of the corrective amplifier is areplica of the line circuit., as for example in the system of Fig. or inthe system of Fig. 2 with the amplifier feedback path made ay replica ofthe line circuit, the -circuit of the amplifier may be made to reduceeffects of non-linearity of the line circuit or cable, forexample cablemodulation due to loading, as, for instance, periodic loading orpermalloy continuous loading, or due to magnetic shielding. Suchmodulation may be troublesome, especially in the case of broad-bandtransmission. It can be shown that, when the cable or line circuit isnonlinear, if the -circuit of the amplifier is made just like the cableand similarly loaded by signals, the modulation of the cable or linecircuit,

referred to the output of the amplifier, is multipliedby y n ll and soisgreatly reduced when p 1. The same methodof compensating for line orcable modulation is readily applicable in systems in which thecompensating line or cable that is just like the transmission line orcable is divided, with the portions forming the -circuits of tandemconnected corrective amplifiers, for example in Fig. 6.

What is claimed is:

l. A signaling system comprising a signal transmission line fortransmitting signals of given 4frequency range, loading elements spacedalong said line for periodically loading said line, an amplifierconnected to receive and amplify signals from said line, and means forpreventing said loading elements from increasing the transmission timeof said line comprising a feed` back path for said amplifier having itscharacteristicls of attenuation versus frequency and phase shift versusfrequency the same as those the transmission time of said linecomprising' al negative feedback path for said amplifier having itstransmission time of smaller order of magnitude than the transmissiontime of said lump loaded line, said path having the sameattenuation-frequency and phase-frequency characterlwithi'oop transferfactor of larger order of magnitude than unity, said path having itstrans- 'mission time of Asmaller order of magnitude than the`'transmission time ci!v said line and consisting essentially of alattice network of one section whose four elements are two impedanceseach equal to the short-circuit impedance of halfthe length of the' lineand two other impedances each equal to the open-circuit impedance ofhalf' the length of the line.

, 4. In combination, a lump loaded signal transmission line whose lengthis many times the wave-length of the lowest 'signal frequency, and anamplifier for receiving and amplifying signals from said line having afeedback path forming therewith a negative feedback loop whose looptransfer factor has a larger order of magnitude than unity, said pathhaving its transmission timeof smaller order of magnitude than thetransmission time of said loaded line and consisting essentially of alattice network of one section whose four elements are twoimpedanceseach equal to the short-circuit impedance of halfl the length of thelump loaded line and two other impedances each equal to the opencircuitimpedance of half the length of the lump loaded line.

5. A signaling system comprising a source of telegraph signals, a loadedtransmission line -for said signals, and an amplifier connected to thereceiving end of said line for amplifying and reshaping the transmittedsignals and increasing their speed of transmission comprising a negativefeedback path with itsA transmission time small compared to that of saidline and with its attenuation-frequency and phase-frequencycharacteristics like those of said line over a frequency range includingthe essential frequency range of the signals, said amplifier with saidfeedback path having its loop transfer factor of larger order ofmagnitude than unity.

6. A signaling system comprising a source of telegraph signals, a loadedcircuit connected to said source for transmitting said signals, anamplifier connected to the receiving end of said circuit for amplifyingthe transmitted signals, a telegraphreceiver for receiving the amplifiedsignals, and means for causing said amplifier to reshape the transmittedsignals and increase the speed of their transmission from said source tosaid receiver comprising a feedback path for said amplifier having thesame indicial admittance as said loaded circuit and forming with saidamplifier a negative feedback loop whose loop transfer constant hasgreater order of magnitude than unity.

'7. A signaling system comprising a source of signals, a loaded circuitconnected thereto producing modulation due to non-linearity of elementsof said circuit, an amplifier connected to the receiving end of saidcircuit for amplifying signals received thereby from said circuit, and anegative feedback path for said amplifier comprising a like loadedcircuit for compensating for said modulation.

8. A wave translating system comprising a signal transmission line, andconnected to said line at thev receiving end thereof, a plurality ofamplifiers in tandem for amplifying signals received thereby from saidline, each of said amunity in the signal frequency range.

HAROLD s. BLACK.

